1. Field of the Invention
This invention relates to a thick-film gas-sensitive element which is suitable for a gas sensor such as an oxygen sensor, and more particularly the invention relates to a thick-film gas-sensitive element having a high resistivity against variation of its operating characteristics due to deterioration of metallic catalyzer on its outer surface layer. The variation in operating characteristics of an oxygen sensor has been a cause of instability of automobile exhaust gas cleaning system, for instance, an instability in that a controlled air-fuel-ratio point in a feedback control system for cleaning engine exhaust gas with ternary catalyzer is shifted to lean side after its durability test.
2. Related Art Statement
The inventors disclosed, in their Japanese Patent Laying-open Specification No. 158346/1985, a titania thick-film gas-sensitive element with 5-30 mole % of platinum-group metallic catalyzer particles dispersed therein. Ensuing studies have shown that the metallic catalyzer in the outer surface layer of the thick film is responsible for certain changes in its performance as a gas-sensitive element. More specifically, when the titania thick-film gas-sensitive element is used in a feedback control system for cleaning automobile exhaust gas with ternary catalyzer, controlled air-fuel-ratio point of the control system has been shifted to lean side after its durability test, and the cause of such shift has been found to be deterioration of the metallic catalyzer particles in the outer surface layer of the thick film.
Various improvements of durability of gas-sensitive thick films have been disclosed in Japanese Patent Laying-open Specifications such as
No. 11226/1978, PA1 No. 130093/1978, PA1 No. 48596/1979, and PA1 No. 106147/1981.
In particular, the last-mentioned Specification taught durability improvement of a pellet type gas-sensitive element by reducing the amount of catalyzer in its outer surface layer as compared with that in its inner portion across electrodes. Although the disclosed improvement contributes to suppression of the above mentioned shift of the controlled air-fuel-ratio point, it does not prevent the troublesome deterioration of the catalyzer particles in the outer surface layer during use. Besides, the reduction of the amount of catalyzer particles is clearly undesirable from the standpoint of the function of trapping poisonous substances, such as lead (Pb) and phosphorus (P), for the purpose of preventing the poisonous substances in exhaust gas from entering into the gas-sensitive element.
As result of a series of studies on the thick-film gas-sensitive element of the above-mentioned Japanese Patent Laying-open Specification No. 158346/1985, the inventors have found that a change in the emission characteristics of the thick film occurs during durability test due to change in the nature or structure of the metallic catalyzer and that such change of the catalyzer mainly depends on adsorption of specific components of the exhaust gas being sensed. The inventors noted a possibility of improvement in the durability of the thick-film gas-sensitive element by minimizing influence of the ambient gas.